Simple matter, complex antimatter, and added strangeness

Continuing our coverage of the antimatter research symposium at AAAS, we …

Having reported on ways to store and use large amounts of simple antimatter—positrons—we'll now turn our attention to more complex forms of antimatter. While creating positrons is a fairly straightforward process, creating more complex interactions between antiparticles in a controlled fashion is a much more complicated task.

The first talk in this part of the symposium looked at the production of the simplest possible anti-element, antihydrogen. Atomic hydrogen is simple, consisting of one electron orbiting a single proton. Its antimatter equivalent is then a positron orbiting an antiproton. The main hurdle to making it is getting enough of each ingredient (positrons and antiprotons) together in the same place for them to react and form an antiatom.

Gerald Gabrielse spoke about his group's work with the ATRAP collaboration in the creation and study of antihydrogen. To accomplish this, they attempted to create an electric potential trap similar to those used in the storage of positrons. However, their traps had to work in two ways simultaneously: they needed a region that would trap the negative antiprotons and a region that would trap the positively charged positrons, yet these regions have to overlap so that collisions can occur and antihydrogen can (potentially) be formed. They accomplished this using a nested Penning trap, and, in 2008, were producing antihydrogen at a rate of about 20 atoms per hour.

While it's cool to say that you can create antiatoms, there's an obvious question: what can you do with them? Gabrielse spoke about using antihydrogen to test the symmetry that exists between matter and antimatter, and how that was violated often enough to give us a matter-dominated Universe. After discovering that P and then CP symmetries were violated by certain natural processes, physicists current believe that CPT symmetry holds—an antiparticle will have the opposite charge, parity, and time symmetry.

Gabrielse believes that, by studying antihydrogen, which should behave identically to hydrogen if CPT symmetry is correct, he can discover any discrepancies that exist between hydrogen and antihydrogen. The ultimate goal being to study the 1s-2s electron transition in both materials, which would provide one of the most detailed tests to date on if and how matter and antimatter differ.

If you are not content with "simple" antihydrogen and want some antimatter with a non-zero strangeness, then you would need to speak with Dr. Zhangbu Xu of the STAR Collaboration about his work creating antihypernuclei. A hypernuclei is a nucleus—usually made up of protons and neutrons—that contains a hyperon, a baryon that contains a strange quark. The example he talked about was 3ΛH which is essentially a tritium atom with one neutron replaced by a Λ particle (a Λ particle consists of up, down, and strange quarks).

In the aftermath of gold-gold collisions carried out at Brookhaven's Relativistic Heavy Ion Collider, Dr. Xu and his colleagues found anti-3ΛH, the heaviest antimatter particle ever created by humans. These particles were seen thanks to their decay path: an anti-3He plus a π+. In addition to finding heavy antimatter, they also found that matter and antimatter—which should be identical in almost every way—are created at different rates. The rate of formation of anti-3He was only half that of 3He, a further route for digging into the reason why our universe is matter-dominated.

To close, Dr. Xu made an overly confident prediction: while currently the heaviest known antimatter particle is the aforementioned anti-3ΛH, he expects that the discovery of a complete anti-α particle will be made in 2011. His demeanor led me to speculate that his team has a strong candidate for an anti-α particle, but that it has not yet been confirmed to a level that makes it ready for public release. Keep an eye on Nobel Intent to see if this prediction holds true.

The ultimate goal being to study the 1s-2s electron transition in both materials [H and anti-H]

Cool, I never really imagined there would be a difference, but I guess it's worth checking!

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His demeanor led me to speculate that his team has a strong candidate for an anti-α particle

Well if they've already got anti-He-3, they're only one anti-neutron shy of an anti-alpha particle, right? Doesn't seem wild to think that extra anti-n could slip in there, with all that goes on the collision of gold atoms.

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an antiparticle will have the opposite charge, parity, and will move opposite in time from its corresponding particles.

(My emphasis) How the heck does that work, anyway? Wouldn't that muck up our ability to measure e.g. the production rates of He-3 vs anti-He-3?

an antiparticle will have the opposite charge, parity, and will move opposite in time from its corresponding particles.

Uh, NO. Obviously antimatter doesn't move backward in time.

The point is that the fundamental laws of nature appear time reversable at the micro-scale -- you can "play a movie" of the "scattering" (collision and interaction) process of fundamental particles and either direction is physically possible. This is a fundamental consequence of the conservation of energy incidentally, or you can see the conservation of energy as a consequence of this time-reversibility ... take your pick.

More fully it is CPT which is believed to be conserved.

Thus an anti-particle with opposite parity cannot be distinguished from the "normal particle."

Even at the QCD scale however entropy applies: the decay of a very energetic particle into a spay of debris particles is theoretically reversible, ... all those particles could come together in the exact manner (but in reverse) to create the energetic particle. But this is extremely unlikely statistically.

The weak force is known to violate parity conservation for several particle/state reactions ... at present it is unknown why ... and there are hints that there are more such parity violations at higher energies ... collectively it is thought to be these parity-violations which result in the predominance of matter over anti-matter, through the CPT symmetry.

We really don't understand parity or the parity breaking mechanisms very well though.

Yeah, the bit about backwards time doesn't make sense to me. How does that fit with thermodynamics?

The referenced Wikipedia article on CPT violations is pretty vague:

The implication of CPT symmetry is that a "mirror-image" of our universe — with all objects having their positions reflected by an imaginary plane (corresponding to a parity inversion), all momenta reversed (corresponding to a time inversion) and with all matter replaced by antimatter (corresponding to a charge inversion)— would evolve under exactly our physical laws.

I'm imagining a universe assembling itself from Hawking radiation into black holes, which then explode to supply the heavy elements which fall into super novas, which convert them into hydrogen and helium using the energy from the background radiation (which conveniently streams into the stars), then the stars evaporate into clouds of gas which the contracting universe packs back into the Singularity at the end of time using cosmic deflation which in turn is erased by a random quantum fluctuation.

CPT symmetry neither requires nor rules out a physical actualization of that symmetry. On the one hand, if a parity inverted antiparticle and a regular particle traveling back in time are indistinguishable, Occam's razor suggests cutting ~half of all particles out of existence is a positive thing. On the other hand, if doing so loses us causality or entropy, it's now making things more complex, calling for a cut in the other direction. CPT symmetry doesn't imply that our universe makes use of that symmetry, but it's a suggestive connection nevertheless.

Without backwards time travel, at time T the scientists collide au + au = anti-he + whatever, then at T+1 the anti-he collides with regular matter (let's say he for simplicity) producing a high energy photon burst.

If the particle is moving in the opposite time direction, at T+1 a high energy photon collides with an he atom and sends it backwards in time, where at T it gets involved in an au + au + he(moving backwards in time) = whatever collision.

The second example seems to violate entropy: why did the he + photon collision happen at exactly the right moment to involve itself in the high energy collision? That's the argument for this symmetry to lack a physical actualization. However, it's worth noting that there could be more particles oscillating back and forth in time then we realize, that it's only when we produce large energy collisions that we jar them out of the oscillation, allowing us to to detect it (the scientists then didn't produce an anti-he, they detected a regular he that was caught bouncing backwards and forward in time)

an antiparticle will have the opposite charge, parity, and will move opposite in time from its corresponding particles.

Uh, NO. Obviously antimatter doesn't move backward in time.

Yeah, wow. Rereading that days after I wrote it makes me wonder what the hell I was thinking when I proof read that and felt it was clear. That is really butchering things on my part, and apologize for any confusion I caused. I'll re-word that once I get home and have a minute.

"Dr. Xu and his colleagues found anti-3ΛHe, the heaviest antimatter particle ever created by humans. These particles were seen thanks to their decay path: an anti-3He plus a π+."

as mentioned by sweaty the decay chain as stated in the article does not respect charge conservation. However it is because Dr Xu and his colleagues (of which I am one!) found anti-3ΛH which does decay to anti-3He plus a π+. Just to be clear an anti-3ΛH (anti)nucleus is a bound state of an anti-proton, anti-neutron and anti-Lambda. The charges of these are -1,0,0 so the total charge is -1. The anti-Lambda decays to anti-proton plus π+, with the anti-proton remaining in the nucleus to form anti-He3 (2 anti-protons, one anti-neutron, total charge -2) and the pion with charge +1 escaping.

"Dr. Xu and his colleagues found anti-3ΛHe, the heaviest antimatter particle ever created by humans. These particles were seen thanks to their decay path: an anti-3He plus a π+."

as mentioned by sweaty the decay chain as stated in the article does not respect charge conservation. However it is because Dr Xu and his colleagues (of which I am one!) found anti-3ΛH which does decay to anti-3He plus a π+. Just to be clear an anti-3ΛH (anti)nucleus is a bound state of an anti-proton, anti-neutron and anti-Lambda. The charges of these are -1,0,0 so the total charge is -1. The anti-Lambda decays to anti-proton plus π+, with the anti-proton remaining in the nucleus to form anti-He3 (2 anti-protons, one anti-neutron, total charge -2) and the pion with charge +1 escaping.

Thanks for the heads up, I was furiously taking notes and clearly jotted it down wrong.

Also, cool research, this session was my favorite of the whole conference.

this is one of the most interesting articles on this type of research I have seen in a few years. It equals the articles (as far as interest in subject matter... so to speak) of cosmology that I have read in the last couple of years. I would LOVE more of this. On a side note, this reminds me a little on the theoretical ultimate computers and the analogies given in relation to blackholes and quazars. THIS reminds me of the Ars of old :) I am happy to see this. Despite mistakes :)

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.